The surface of the hot-rolled coil is usually blue-gray, and the surface is smooth and has a certain luster. However, due to the different chemical composition and rolling process of different steel grades, sometimes red iron oxide scale (commonly known as red rust) will appear on the surface of the steel plate, which not only affects the appearance of the product but also increases the wear and tear of the rolls. In the process of hot rolling, iron oxide scale mainly composed of FeO is basically formed on the surface of the strip. FeO has high plasticity under higher temperature conditions and can deform with the matrix without breaking.
However, during low-temperature rolling, FeO will be broken, so that the specific surface area in contact with the air will increase, which will continue to be oxidized to Fe2O3.
1 Test steel chemical composition
Analysis of Red Iron Oxide Defects The defect covers the entire surface of the steel plate, elongates along the rolling direction, and has obvious directionality. There are obvious pits after pickling in some positions, and the thicker the specification, the more serious the defect, the red iron oxide scale on the surface of the hot coil. The defect is shown in the picture.
Through analysis, it is found that the structure of the iron oxide scale of this material is relatively complex, and the interface between the iron oxide scale and the substrate has pits, indicating that the iron oxide scale is indented. According to the analysis results, it is inferred that the cause of the red rust defect may be related to Si. In order to further confirm the corresponding relationship between the defect and the surface quality of the slab, the slab was tracked, and it was found through observation that there were black plaque defects on the surface of the slab after being descaled in the furnace.
In order to analyze the influence of the “black spot” of the slab in the heating furnace on the iron oxide scale, the slab was removed after descaling after passing through the furnace, and the iron oxide scale on the surface of the slab was analyzed by scanning electron microscope. analyze. Through analysis, it is determined that the types of iron oxide scale are FeO and FeSiO4. It can be seen that there is a corresponding relationship between the red iron oxide scale defect and the “black spot” defect on the slab surface. In order to remove this defect, it is necessary to focus on controlling the black spot on the slab surface.
2 Red oxide skin defect control measures
2.1 Formation mechanism of hot-rolled iron oxide scale
The formation process of the iron oxide scale is the diffusion process of two elements, iron and oxygen. Oxygen diffuses from the surface to the inside of the iron, while iron diffuses to the outside. In the oxidation reaction, the concentration of oxygen in the outer layer is large, the concentration of iron is small, high-valent oxides of iron are formed, and the concentration of iron in the inner layer is large, and the concentration of oxygen is small, and low-valent oxides of oxygen are formed.
2.2 Influence of heating temperature on red iron oxide scale
According to the literature, the most effective way to eliminate or reduce the iron oxide scale defects on the surface of the slab is to increase the tapping temperature, so that the surface temperature of the slab is higher than the melting point of FeSiO4 when the slab is descaled after the furnace, making it liquid. According to the state diagram of the FeO-SiO2 system, it is necessary to ensure that FeSiO4 (fayalite) in the slab is completely liquid, and the furnace temperature should be controlled above 1205°C. For this reason, under the premise of ensuring the inlet temperature of finishing rolling, it is necessary to increase the descaling pass and reduce the descaling speed of the pass as much as possible.
2.3 Influence of finishing temperature and rolling speed on red iron sheet
According to on-site observation, it is found that the red iron oxide scale of thin-gauge products has fewer defects. Through the comparison of hot-rolling processes with different thicknesses, the finish rolling inlet temperature of steel plates with thicknesses greater than 3.0mm is reduced by 30°C, and the final rolling temperature is increased by 10°C at the same time. The rolling speed is increased by 2m/s, thereby reducing the contact time between the coil and the air before coiling, and the red oxide scale defects are significantly reduced by on-site observation.
2.4 Influence of laminar cooling and coiling process on red iron sheet
According to the reaction formula of H2O and steel below, it can be seen that Fe2O3 will not be generated in the steel plate during the layer cooling process, because the surface of the steel plate is covered with a film before coiling, which prevents the oxygen in the air from contacting the steel plate, which is beneficial to prevent the occurrence of red Iron oxide scale. The reaction between H2O and steel is as follows: Since the steel plate still reacts with oxygen in the air after coiling, the proportion of Fe2O3 in the reaction product is relatively large. In order to reduce the contact area between the steel plate and oxygen, the coiling process adopts a high-tension coiling process, reduces the gaps in each layer of the steel coil, effectively reducing the amount of air entering, and reduces the defects of red iron oxide scale.
3 Conclusions
(1) The red rust defect on the surface of the hot-rolled sheet is due to the high proportion of Fe2O3 in the iron oxide scale, and its essence is that FeSiO4 adheres to the steel matrix and FeO, which leads to oxidation to red oxidation in contact with cooling water and air during hot rolling and cooling. tin.
(2) Ensure that the descaling temperature after the slab furnace is above 1205 °C so that FeSiO4 is in a molten state, which is the most effective method to remove FeSiO4 on the surface of the slab.
(3) Increasing the rough rolling descaling pass and reducing the descaling speed can remove FeO on the surface of the steel plate and reduce the subsequent generation of red scale.
(4) For the erythema defect on the surface of the thick-gauge steel plate, it is possible to reduce the finishing rolling inlet temperature and increase the finishing rolling temperature, thereby increasing the finishing rolling speed.
(5) Appropriately increase the coiling tension, reduce the gap between each layer of the coil after coiling, and reduce the amount of air entering, which can reduce the defects of the red iron oxide scale.
